Device for preparing a dispersion of water-soluble polymers in water, and method implementing the device

ABSTRACT

Device for dispersing a water-soluble polymer comprising: a chamber for grinding and draining of the dispersed polymer comprising a rotor and a stator, and on all or part of the periphery of the chamber, a ring fed by a secondary water circuit. The ring communicates with the chamber for spraying of pressurised water on the blades of the stator. A method implementing the device is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of French application No. 0758252 filedOct. 12, 2007, and U.S. provisional application No. 60/982,250 filedOct. 24, 2007, the full disclosures of which are hereby incorporated byreference herein.

BACKGROUND ART

The present invention relates to an installation for the wet grinding ofwater-soluble polymers to obtain their dispersions and thensubsequently, their rapid dissolution in water. It also relates to amethod implementing the device.

According to the invention, the polymer is put in suspension in thewater by passing through a grinder comprising a rotor with knivesrotating in a stator with very close blades; the plugging of this statorby the polymer gel formed being prevented by secondary water jets whichdisperse and dilute the polymer.

Among the water-soluble polymers belonging to the prior art, partiallyhydrolysed acrylamide polymers and their copolymers are particularlyknown, and also xanthan gums, cellulose derivatives and guar gums. Thesepolymers develop a viscosity thanks to their molecular weight and/or theinter-chain ionic repulsions. The mechanism governing the viscosity islinked to a rise in hydrodynamic volume or to inter-chain repulsions.

Although acrylamide (co)polymers are usually available commercially inthe form of powders, they are generally used in dilute aqueous solutionsin industrial applications. This necessitates a step of dissolution ofthe polymer in the water in precise physical and chemical conditions.

However, even if these polymers are hydrophilic, their dissolution isdifficult. Their dissolution varies, in particular, according to theircomposition and their molecular weight.

Thus, to be used in solution, powder polymers are first dispersed inwater using wetting equipment. The main equipment used is of varioustypes:

-   -   eductor with dry or wet feed hopper,    -   water/air disperser in which the polymer is transported by an        air actuated system in a chamber where it is wet by spray        nozzles,    -   various high speed stirrers.

The dispersion in water thereby obtained is then dissolved continuouslyor in batches by stirring.

The main drawback of these dispersion systems is that, the higher themolecular weight of the polymer, the higher the viscosity of theresulting solution. This has the consequence of limiting the polymerconcentration in the water, generally from 1 to 5 grams/litre, andtherefore requiring extremely large dissolution tanks for large scaleindustrial applications.

For example, for high molecular weight acrylamide (co)polymers (about 15million), in powder form with an average particle size of 0 to 1 mm, atthe temperature of 20° C., the dissolution time required to obtain asolution of 5 g/l is about:

-   -   4 hours for a nonionic polymer,    -   1 hour for an anionic polymer,    -   45 minutes for a cationic polymer.

To solve these problems of concentration, dispersion/dissolution andequipment cost/size, various methods have been developed. The mainmethods are listed below. They are based on two guidelines:—modificationof the commercial form of the polymer (cf 1-5) and—improvement of thedissolution equipment (cf 6).

1/Reduction of the Size of Particles with Standard Grain SizeDistribution by Dry Grinding.

Acrylamide (co)polymers in solid (powder) form are mainly produced bygel polymerisation followed by steps of chopping, drying then grinding.It is well known to a person skilled in the art that a significantaction on the grain size distribution of the powder (decrease) has theresult of facilitating its hydration and hence its dissolution.

However, this solution has many limiting factors, that is:

-   -   high grinding cost: because the softening point is low (close to        50° C.) requiring the use of large grinders with large        quantities of cooling air or the use of cryogenic systems,    -   a very dusty end product: since the product is used directly by        the operators, it is difficult to control the rate of particles        in the atmosphere without sophisticated equipment,    -   an increase in ‘fisheyes’: during their use, the fine polymeric        particles tend to cake together on wetting by forming numerous        ‘fisheyes’. These are gelatinous particles ranging in size from        a few millimetres to a few centimetres, and which only dissolve        after several hours to several days. These aggregates tend to        plug the lines, the metering pumps and the filters.

2/Inverse Emulsion (Water-in-Oil) Polymerisation

In that well known process to a person skilled in the art, the monomersare emulsified by a hydrocarbon and polymerised in the presence ofstabilising surfactants. To dissolve the polymer thus obtained, it isnecessary to add an inverted surfactant (high HLB) either directly tothe emulsion or during its dissolution. The end product therefore has ahigh concentration of detrimental surfactants and an organic phase,resulting in a significant additional cost of raw materials (30 to 60%),transport (30 to 40%) and storage. This means that the emulsions arewidely used for low- or medium-consumption applications because of theirease of use, but are too expensive for large scale projects.

3/Aqueous Dispersion Polymerisation (Also Called ‘Water-in-WaterEmulsion’)

This technique consists in polymerising a monomer or a mixture ofmonomers in water containing a salt and/or other chemical agents insolution or in dispersion. The hydrophilic polymer formed during thepolymerisation precipitates when it reaches a sufficiently highmolecular weight. At the end of the polymerisation, a liquid dispersionof polymer particles in suspension in the aqueous mixture is recovered.The advantages of this technology are obvious. As to their manufacturingcost, this remains low, that is, similar to that of powder polymers,because the dispersion obtained comprises almost exclusively polymer,water and salts. Moreover, it has the same decisive advantage as theoil-in-water type emulsion, that is, very rapid solubilisation of thepolymer in water.

These products nevertheless face several obstacles to their development:

-   -   a low concentration (15 to 20%) and hence extra cost for        transport and storage,    -   limited polymer molecular weight,    -   reduced shelf life.

4/Suspension Polymerisation

This polymerisation method consists in forming droplets of an aqueoussolution of the monomer or monomers in suspension in an inert liquidwhich, after polymerisation by addition of a catalyst, yield polymers inthe form of beads. At the end of polymerisation, the water is thenremoved during an azeotropic distillation step. The polymer beads arethen filtered and dried. The azeotropic distillation step is generallyconsidered as critical. With this method, the particle size (100 to 400microns) can be reduced nearly uniformly without forming largequantities of fines.

Here also, this solution has many limiting factors, that is:

-   -   the polymeric particles formed by this method also have a strong        tendency to form fisheyes above a certain concentration,    -   moreover, the major limit of this method is the inability to        produce very high molecular weights through its use. The        molecular weights of the resulting polymers are limited to 10-12        million, which is insufficient in many industries.

5/Placing the Powder in Suspension in Surfactants

The finely ground polymer powder can be placed in suspension either in ahydrocarbon containing large quantities of surfactants, or directly inpure surfactants. These suspensions are rapidly dissolved but areunstable and have the same economic drawbacks as reverse emulsions.

6/Wet Grinding of Polymer Powder Having a Standard Grain SizeDistribution

The standard grain size distribution polymer is placed in suspension inthe water and then ground. To do this, documents U.S. Pat. No.4,845,192, U.S. Pat. No. 4,877,588 and U.S. Pat. No. 4,529,794 describea device comprising a closed cage equipped with fixed and moving knives(mounted on a rotor) and positioned at a spacing of 50 to 500 microns,with a clearance of 50 to 500 microns, which cut the product into verysmall particles, typically smaller than 200 microns. This apparatus ismanufactured by URSCHEL under the trade name Comitrol. The Comitrol 1500has a cutting diameter of 200 mm.

According to this method, it is the cutting dimension that determinesthe final dissolution time. Hence this method, which provides asignificant improvement in dissolution time of water-soluble polymersafter dispersion in the device appears to be advantageous. However, ithas many major drawbacks:

-   -   the spacing of the knives and their angle is critical for        obtaining a satisfactory cutting,    -   the speed required for satisfactory operation is very high: 10        000 to 13 000 rpm (e.g.: Comitrol 1500 apparatus equipped with        an 8-inch rotor). At lower speed of rotation of the rotor, the        system is blocked by plugging of the interval between the fixed        knives: no dissolution is then possible,    -   the wear of the fixed and mobile knives is extremely rapid. On        average, after continuous in-line use, it has been found that        the knives had to be replaced every 10 to 90 days. This has the        consequence of requiring the doubling of the number of grinding        apparatus necessary and requiring difficult and lengthy        maintenance due to the replacement of about 200 knives in very        precise conditions and often beyond the scope of local        maintenance personnel. This aging also occurs when using high        strength materials,    -   furthermore, at these speeds, rapid aging and overheating of the        bearings are observed, making this apparatus incompatible with        ATEX standards (relative to workplace equipment safety). Ceramic        bearings could diminish the problem without solving it,    -   finally, the installed motor capacities are extremely high, for        example 30 kW for a Comitrol 1500.

Due to these drawbacks which appear prohibitive, in 20 years, the use ofthis type of apparatus for dispersing water-soluble polymers has notspread.

The invention overcomes all the above mentioned drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, it has been found surprisingly that it ispossible to disperse water-soluble polymers in water using a wetgrinding unit rotating at industrial speed, about 20 to 40 m/speripheral speed (with an average speed of 3000 rpm for a rotor diameterof 200 mm), and having a high flow rate (for example, 300 kg/h for agrinder equipped with a 200 mm diameter rotor), by prewetted thestandard grain size polymer with a first volume of water, and thenchopping it immediately, before dispersing it by the addition of asecond volume of water.

The subject of the invention is a device for dispersing a water-solublepolymer having a standard grain size distribution of between 0.15 and 1mm comprising:

-   -   a wetting cone in which the polymer is metered, usually using a        metering screw, the said cone being connected to a primary water        inlet circuit,    -   at the bottom end of the cone:        -   a chamber for grinding and draining of the dispersed polymer            comprising:            -   a rotor driven by a motor and equipped with knives                optionally tilted with respect to the radius of the                rotor,            -   a fixed stator consisting of blades optionally tilted                with respect to the radius of the rotor and uniformly                spaced,            -   the rotor/stator assembly for wet grinding the polymer,        -   on all or part of the periphery of the chamber, a ring fed            by a secondary water circuit, the ring communicating with            the chamber for the spraying of pressurised water on the            blades of the stator and thereby releasing the ground and            swollen polymer on the surface of the said blades,        -   the assembly serving to reduce the speed of rotation and            increase the concentration of the dispersion at the outlet            of the grinding chamber.

According to a first feature, the polymer is wet in the cone byoverflow, the cone being equipped in this case with a double jacket atthe base of which the primary water inlet circuit is connected.Alternately, this wetting can also take place in a cone by any othermeans well known to a person skilled in the art, for example spraynozzles or a flat jet.

In practice, the rotor is equipped with 2 to 20 knives, advantageouslybetween 4 and 12. However, depending on the rotor diameter, the numberof knives may vary. Similarly, the number of blades of the stator isvariable according to the diameter thereof. In practice, it is between50 and 300, advantageously between 90 and 200 for a rotor diameter of200 mm. Moreover, and according to another feature, the knives areoptionally more or less tilted with respect to the radius of the rotor.Advantageously, this tilt is between 0 and 15°, preferably between 2 and10°.

According to another feature, the distance between the blades of thestator is between 50 and 800 microns. For effective grinding, thedistance between the knives of the rotor and the blades of the stator isbetween 50 and 300 microns, advantageously between 100 and 200 microns,in practice about 100 microns. Advantageously, the blades of the statorare tilted at an angle smaller than 10° with respect to the radius ofthe rotor. These blades are either assembled in a casing, or cut in themass of a metal or of a high strength compound.

In one advantageous embodiment, the rotor knives are not tilted, whilethe stator blades are tilted.

Furthermore, concerning the peripheral ring, it communicates with thegrinding and draining chamber via perforations in the form of holes,slits or equivalent, whereof the size and distribution on the ring aresuch that the secondary water can be propelled on the blades of thestator at a pressure serving to prevent the clogging by the gelledpolymer, of the spaces between the blades. Accordingly, the pressureapplied by the rotor pump effect can be sharply decreased without a riskof plugging. The smaller the spacing of the blades, the higher thepressure required for continuous operation.

A further subject of the invention is a method for dispersing awater-soluble polymer, particularly acrylamide and/or methacrylamide(co)polymer using the abovementioned device.

Practically, the molecular weight of the polymer is of at least 10millions, advantageously more that 15 M.

According to this method, continuously or in batches:

-   -   the polymer is prewetted in the wetting cone by a quantity of        primary water suitable for obtaining a polymer suspension having        a concentration of 15 to 100 g/l, advantageously 20 to 80 g/l,    -   then, instantaneously, in the grinding and draining chamber, the        size of the prewetted polymer is reduced, in practice to a size        of 50 to 200 microns by chopping the polymer between the knives        of the rotor and the blades of the stator, without degradation        of the molecular weight of the polymer,    -   then, the pressurised secondary water from the peripheral ring        is used to clear the interstices between the blades of the        stator in which the swollen polymer is liable to be fixed,    -   the dispersed polymer is removed, having, by dilution with the        secondary water, a concentration of between 3 and 30 g/litre,        advantageously between 10 and 25 g/l.

According to an essential feature, the conformation of the device, inproviding for a wetting step by a first volume of water leading to adispersion of the polymer followed by a step of dilution of thedispersed polymer, with a second volume of water allows to considerablyreduce the speed of rotation of the rotor.

Advantageously, the primary water represents between 20 to 40% by weightof the total water (primary water+secondary water) whereas secondarywater represents between 60 to 80% of the total water (primarywater+secondary water) which is required for obtaining a polymerconcentration of between 3 and 30 g/litre.

Thus and according to one feature of the method, the speed of rotationof the rotor is between 2000 and 5000 rpm, on average about 3000 rpm fora cutting diameter of 200 mm. It is between 3000 and 6000 rpm for acutting diameter of 10 cm and between 1500 and 3000 rpm for a cuttingdiameter of 40 cm. More generally, according to the diameter of therotor also referred to as cutting diameter, the rotor speed is between20 and 40 m/s instead of 90 to 150 m/s for a Comitrol 1500 apparatus.

Furthermore, to avoid plugging the space between the stator blades bythe ground polymer, the secondary water is propelled through theperforations of the ring at a pressure of at least 1 bar, usually at themain water pressure, that is 3 to 6 bar or more, for very fineintervals, in general between 1 and 10 bar.

As already stated, it is essential, in the present invention, toprewetted the polymer before its chopping in proportions of between 20and 40% of the total quantity of water required for its dilution at aconcentration of between 3 and 30 g/litre. This allows to sharplyincrease the flowrate of the apparatus, which can be up to 300 kg ofpolymer per hour for a cutting diameter of 200 mm, by dividing themethod into two steps: prewetted followed by final dilution duringgrinding.

In view of these conditions of implementation, there is no specificrequirement associated with the wear of the device, the maintenanceperiod of the grinding unit being longer than one year and typicallythree years.

The grinding unit of the invention can be applicable to all productswith standard grain size distribution having a substantial hydrophiliccharacter such as: cellulose salt or ether, xanthan gum, guar gum, etc.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The invention and its advantages appear clearly from the followingexamples, in conjunction with the figures appended hereto.

FIG. 1 is a schematic side view of the device of the invention.

FIG. 2 is a cross-section along AA′.

DETAIL DESCRIPTION Example 1 The Device

According to FIG. 1, the device of the invention comprises:

-   -   a wetting cone (1) connected at its apex to a column (2)        batching the standard grain size distribution polymer, usually        via a metering screw, the cone (1) being connected at its bottom        to a primary water inlet circuit (3) which feeds an overflow (4,        4′),    -   at the bottom end of the cone, an assembly (5) comprising:        -   a chamber (6) for grinding and draining of the dispersed            polymer (FIG. 2) comprising:            -   a rotor (7) driven by a motor (8) fitted with knives                (9),            -   a fixed stator (10) comprising blades (11) uniformly                spaced and slightly tilted with respect to the radius of                the rotor,        -   on all or part of the periphery of the chamber, a ring (12)            fed by a secondary water circuit (13), the ring (12)            communicating with the chamber (6) via slits (14) for            spraying pressurised water on the blades (11) of the stator            (10).

Example 2 Application

Three grinding units according to the invention having different sizeswere tested in this example.

The technical features and dissolution conditions are given in the tablebelow using an acrylamide/sodium acrylate copolymer having a molecularweight of 19 million and grain size distribution of 0-1000 microns.

Test 1 Test 2 Test 3 ‘PSU 300’ ‘PSU 100’ ‘PSU 1000’ Technical featuresof the grinding unit Cutting diameter in mm (rotor 200  100  400 size)Number of fixed knives 90 50 200 Height of fixed knives in mm   16.6  16.6   33.2 Spacing between knives 300 microns 200 microns 400 micronsSpacing between fixed/mobile 100 microns 100 microns 100 microns knivesCutting angle 3° 2° 3° Number of mobile knives (i.e.: on  6  4  12 therotor) Rotor speed 3000 rpm 5000 rpm 2000 rpm Rotor power 7.5 kW 3 kW 20kW Dispersion characteristics Primary water flow rate 10 m3/h 3 m3/h 20m3/h 25° c. 25° c. 25° c. Anionic polyacryl amide flow rate 300 kg/h 110kg/h 1400 kg/h (anionicity 30%; molecular weight 19 million; grain sizedistribution 0-1000 microns) Secondary water flow rate in the 20 m3/h 8m3/h 40 m3/h concentric ring surrounding the 25° c. 25° c. 25° c. stator(pressure 4 bar) Final concentration of dispersion 10 gr/l 10 gr/l 23.3gr/l Final pressure 1.5 bar 1 bar 1.8 bar Dissolution time to obtain the<10 min <5 min <5 min usual maximum viscosity

As may be observed, the grinding unit of the invention allows to:

-   -   obtain very short dissolution times at high concentration even        for very high molecular weight polymers, without the formation        of fisheyes and requiring no subsequent filtration,    -   deliver these dispersions under pressure, which allows to        transport them to the end use; if necessary, the grinding units        can also be connected directly to a positive displacement pump        of the same capacity (Moyno, gear, lobe, screw, type etc.) for        longer distances,    -   operate continuously over very long periods without damage to        the drive bearings, nor to the rotor or the stator, the        temperature of the ball bearings only rising by a few degrees (5        to 10° C.) and then being stabilised,    -   and constructing apparatus of different sizes and therefore        adaptable to the demand while maintaining normal industrial        speeds (contrary, for example, to the URSCHEL equipment which,        in case of a small unit similar to the ‘PSU 100’ would demand        the use of excessively high speeds of about 20000 to 30000 rpm).

1. Device for dispersing a water-soluble polymer having a standard grainsize distribution of between 0.15 and 1 mm comprising: a wetting cone inwhich the polymer is metered, said cone being connected to a primarywater inlet circuit, at a bottom end of the cone: a chamber for grindingand draining of the dispersed polymer comprising: a rotor driven by amotor and equipped with knives, a fixed stator having blades, on all orpart of a periphery of the chamber, a ring fed by a secondary watercircuit, the ring communicating with the chamber for spraying ofpressurised water on the blades of the stator and thereby releasingground and swollen polymer on a surface of said blades.
 2. Deviceaccording to claim 1, wherein the knives are tilted with respect to aradius of the rotor.
 3. Device according to claim 2, wherein the knivesare tilted by an angle of between 0 and 15° with respect to the radiusof the rotor.
 4. Device according to claim 2, wherein the knives aretilted by an angle between 2 and 10° with respect to the radius of therotor.
 5. Device according to claim 1, wherein the blades are tiltedwith respect to a radius of the rotor and uniformly spaced.
 6. Deviceaccording to claim 1, wherein a first distance between the blades of thestator is between 50 and 800 microns, while a second distance betweenthe knives of the rotor and the blades of the stator is between 50 and300 microns.
 7. Device according to claim 6, wherein said seconddistance is between 100 and 200 microns.
 8. Method for dispersing awater-soluble polymer having a standard grain size distribution ofbetween 0.15 and 1 mm implementing the device according to claim
 1. 9.Method according to claim 8, wherein, in continuous or batch mode: thepolymer is prewetted in the wetting cone by a quantity of primary watersuitable for obtaining a polymer suspension having a concentration of 15to 100 g/l, then, instantaneously, in the grinding and draining chamber,size of the prewetted polymer is reduced by chopping the polymer betweenthe knives of the rotor and the blades of the stator, then, thepressurised secondary water from the peripheral ring is used to clearinterstices between the blades of the stator in which the swollenpolymer is liable to be fixed, and the dispersed polymer is removed,having, by dilution with the secondary water, a concentration of between3 and 30 g/litre.
 10. Method according to claim 9, the concentration bydilution with the secondary water is between 10 and 25 g/l.
 11. Methodaccording to claim 9, wherein the primary water represents between 20 to40% by weight of total water, total water comprises the primary waterand the secondary water, and the secondary water represents between 60to 80% of the total water.
 12. Method according to claim 9, whereinperipheral speed of rotation of the rotor is between 20 and 40 m/s. 13.Method according to claim 9, wherein the secondary water is propelledunder a pressure of between 1 and 10 bar.
 14. Method according to claim13, wherein the secondary water is propelled under a pressure of between3 and 6 bar.
 15. Method according to claim 9, wherein the water-solublepolymer comprises a (co)polymer of acrylamide and/or methacrylamide.